The further resolvability of a great many double stars is perhaps the most curious result of modern improvements in the optical means of observing them. With every addition to the defining power of telescopes, the visible complexity of stellar systems has increased so rapidly as to inspire a suspicion that simple binary combinations may be an exception rather than the rule. The frequency with which what appeared to be such have yielded to the disintegrating scrutiny of Mr. Burnham and others, suggests at any rate the presence of an innate tendency, and seems to show that the duplicity of stars is no accident of nebular condensation, but belongs essentially to the primitive design of their organisation. Although we can never become fully acquainted with all the detailed arrangements of stellar systems, we are then led to suppose them far more elaborate and varied than appears at first sight. Each, we cannot doubt, is adapted by exquisite contrivances to its special end, reflecting, in its untold harmonies of adjustment, the Supreme Wisdom from which they emanate.

The continuance of the process of optical dissociation, begun by the splitting-up of an apparently simple star, sometimes shows the primary, sometimes the satellite, not unfrequently both primary and satellite, to be very closely double. Ternary systems are accordingly of two kinds. In one, the smaller star consists of two in mutual circulation, and concurrent revolution round a single governing body; in the other, an intimately conjoined pair guides the movements of an unattended attendant.

The Milky Way shows to the naked eye as a vast, zoneshaped nebula; but is resolved, with very slight optical assistance, into innumerable small stars. Its stellar constitution, already conjectured by Democritus, was, in fact, one of Galileo's earliest telescopic discoveries. The general course of the formation, however, can only be traced through the perception of the cloudy effect impaired by the application even of an opera-glass. Rendered the more arduous by this very circumstance, its detailed study demands exceptional eyesight, improved by assiduous practice in catching fine gradations of light. Our situation, too, in the galactic plane is the most disadvantageous possible for purposes of survey. Groups behind groups, systems upon systems, streams, sheets, lines, knots of stars, indefinitely far apart in space, may all be projected without distinction upon the same sky-ground. Unawares, our visual ray sounds endless depths, and brings back only simultaneous information about the successive objects met with. We are thus presented with a flat picture totally devoid of perspective-indications. Only by a long series of inductions (if at all) can we hope to arrange the features of the landscape according to their proper relations.

To the uncritical imagination, the Milky Way represents a sort of glorified track through the skies—

The stars differ obviously in colour. Three or four among the brightest strike the eye by their ardent glow, others are tinged with yellow, and the white light of several has a bluish gleam like that of polished steel. Reddish tints are, however, in the few cases in which they affect lucid stars, the most noticeable, and were the only ones noticed by the ancients.

Ptolemy designates as ‘fiery red’ (ὑπόκιῤῥοι) the following six stars: Aldebaran, Arcturus, Betelgeux, Antares, Pollux, and—mirabile dictu—Sirius! all the rest being indiscriminately classed as ‘yellow’ (ξάνθοι). Now Pollux at present, though by no means red, is at least yellowish, but Sirius is undeniably white with a cast of blue. A marked change in its colour since the Alexandrian epoch might thus at first sight appear certain, the more so that Seneca makes express mention of the dog-star as being ‘redder than Mars;’ Horace has ‘rubra Canicula’ as typical of the heat of summer; and Cicero, in his translation of Aratus, speaks of its ‘ruddy light.’ Nevertheless the case is doubtful. The questionable epithet, in all probability, crept into the ‘Almagest’ by a transcriber's error, Ptolemy not being responsible for it. In the early Arabic versions of that work it evidently did not occur, for Arab astronomers of the tenth and subsequent centuries ignored the imputation of colour to the dog star, and Albategnius stated the number of Ptolemy's red stars as five.

The facts connected with the light-changes of stars are in the highest degree strange and surprising; and wonder is not lessened by our daily-growing familiarity with them. They are of everyday occurrence, they can be predicted beforehand, in many cases with nearly as close accuracy as an eclipse of the sun or moon, and they affect in manifold ways a great number of objects. Stellar variability is of every kind and degree. With the regularity of clockwork some stars lose and regain a fixed proportion of their light; others show fitful accessions of luminosity succeeded by equally fitful relapses into obscurity; many waver, in appearance lawlessly, about a datum-level of lustre itself perhaps slowly rising or sinking. The rule of change of a great number is that of an evident, though strongly disturbed periodicity; a few seem to spend all their powers of shining in one amazing outburst, after which they return to their pristine invisibility or insignificance.

The amount is as much diversified as the manner of fluctuation. Changes of brightness so minute as almost to defy detection are linked on by a succession of graduated examples to conflagrations in which emissive intensity is multiplied a thousand times or more in a few hours. The range of variation is in some stars sensibly uniform; they subside during each crisis of change to the same precise point of dimness, and recover, without diminution or excess, just so much light as they had before.

When all the stars blaze out on a clear, moonless night, it seems as if it would be impossible to count them; and yet it is seldom that more than 2,000 are visible together to the unaided eye. The number, however, depends very much upon climate and sharpness of sight. Argelander enumerated at Bonn, where rather more than eight-tenths of the sphere come successively into view, 3,237 stars. But of these no more than 2,000 could be, at any one time, above the horizon, and so many would not be visibly above it, owing to the quenching power of the air in its neighbourhood. Heis, at Münster, saw 1,445 stars more than Argelander at Bonn; Houzeau recorded 5,719 at Jamaica; Gould 10,649 at Cordoba in South America. The discrepancies of these figures, setting aside the comparatively slight effect of the increased area of the heavens displayed in low latitudes, are due to the multitude of small stars always, it might be said, hovering on the verge of visibility. If, indeed, the atmosphere could be wholly withdrawn, fully 25,000 stars would, according to a trustworthy estimate, become apparent to moderately good eyes.

Our system of designating the stars has come down to us from a hoar antiquity. It is a highly incommodious one. ‘The constellations,’ Sir John Herschel remarks, ‘seem to have been almost purposely named and delineated to cause as much confusion and inconvenience as possible.